1. Field of the Invention
The invention relates to an integrated circuit having a load, and more particularly, to a two-terminal protecting circuit for protecting a light emitting device with back-flow and open-circuit protection, and an integrated circuit integrating the light emitting device and protecting circuit.
2. Description of the Prior Art
Light emitting diodes (LEDs) have found a myriad of applications in many electrical circuits produced for consumer, commercial, industrial and military uses. LEDs are semiconductor devices that convert electrical energy directly into light, and, like many other electrical components, are susceptible to damage or destruction when exposed to excessive currents or voltages. When the need arises, circuits can be designed to provide protection to devices that may encounter the over-currents and over-voltages. Generally speaking, as the sophistication of the protection circuits increase, the cost and reliability are adversely affected.
LEDs are often used as light indicators or other light sources for portable electronic devices such as mobile phones, notebook computers, and personal digital assistants (PDAs). However, there have been increasing demands for LEDs to be applied to larger displays such as large neon signs. Such applications require many LEDs for producing a sufficient amount of light. Since the forward-biased current of an LED increases exponentially with the applied forward-biased voltage, it is desirable to drive LEDs with current sources to achieve matching luminance of different LEDs. Please refer to FIG. 1 for a diagram of a prior art LED circuit 10 used for a large-size display. The LED circuit 10 includes a plurality of light emitting diodes LED1-LEDn, an applied voltage VDD, and a current source Is. The LED1-LEDn are coupled in parallel and driven with the same current source Is. The sum of the current passing through each LED is equal to the current provided by the current source Is. Therefore, if one of the LEDs in the LED circuit 10 fails, its current will be shunted to the other LEDs. The more devices among LED1-LEDn that fail, the more the rest of the functioning LEDs are prone to fail due to increased passing current. This kind of avalanche failure greatly shortens the lifetime of the LED circuit 10.
Please refer to FIG. 2 for a diagram of another prior art LED circuit 20 for a large-size display. The LED circuit 20 includes a plurality of light emitting diodes LED1-LEDn, an applied voltage VDD, and a plurality of current sources Is1-Isn. The LED1-LEDn are each coupled in series with the current sources Is1-Isn, respectively, and each of the LED1-LEDn and its corresponding current source Is1-Isn are coupled in parallel. Unlike the prior art LED circuit 10, each of the LEDs is driven with its own respective current source. In the LED circuit 20, a failing LED does not influence other functioning LEDs. Therefore, the LED circuit 20 can solve the problem encountered by the LED circuit 10. However, since each of the LEDs requires a separate current source, the LED circuit 20 has a complicated structure and is very costly. Additionally, current provided by different current sources also varies, and this results in a different luminous effect from each LED. The LED circuit 20 is costly, complicated, and cannot achieve uniform luminance.
Please refer to FIG. 3 for a diagram of another prior art LED circuit 30 for a large-size display. The LED circuit 30 includes a plurality of light emitting diodes LED1-LEDn and a current source Is. The LED1-LEDn and the current source Is are coupled in series. The LED circuit 30 has a simple structure and only requires one current source. Due to the nature of the series connection, the current passing through each LED is the same, and this results in uniform luminance of each LED. However, if one of the LEDs fails, the current path will be blocked and the LED circuit 30 will not be able to function even if the rest of the LEDs still work. The LED circuit 30 has zero margin for failure of a device and is hardly practical in real applications.
Please refer to FIG. 4 for a diagram of another prior art LED circuit 40 for a large-size display. The LED circuit 40 adopts a plurality of back-flow preventing Zener diodes ZE1-ZEn in parallel with the LED1-LEDn for protecting the LEDs from a surge, such as an ESD surge. For example, when a voltage Vd is established between the two terminals of the LED1, the I-V curve of the Zener diode ZE1 is shown in FIG. 5. When the voltage Vd drops below a voltage Vf of the Zener diode ZE1, the back-flow current, shunted from the LED 1, flows through the Zener diode ZE1, preventing the LED1 from damage. However, Zener diodes ZE1-ZEn only protect the LED1-LEDn from breakdown due to reverse currents passing from the cathodes to the anodes of the LEDs and cannot prevent one failing LED from blocking the forward current.